"Our jets took off and started bombing. And now the ground operation is underway," Turkish President Erdogan said.

Turkey's aerial and ground forces entered into Syria battling Kurdish militia in the Afrin region under the offensive Ankara calls "Olive Branch Operation". Troops crossed the border and pushed into northern Syria to target the YPG fighters, who Turkey labels as "terrorists".

This opens a new front in the long-running Syrian civil war as the Damascus regime earlier warned the intruding Turkish jets would be shot down. The Syrian Kurdish group, YPG – which is backed by the US in the fight against Islamic State (Isis) – said it has retaliated to Turkey's stepped up military campaign.

"Our units have entered Afrin from two branches at 11:05 a.m. with the Free Syrian Army [FSA]. This means the land operation has begun," said Turkey's Prime Minister Binali Yıldırım adding that there are anywhere between 8,000 and 10,000 "terrorists" in Afrin province.

Ankara announced the formal start of the "Olive Branch Operation" by ground troops after a round of major air strikes by Turkish jets on 20 January. Thirty-two jets took part on the first day of onslaught destroying dozens of Kurdish positions and interests.

Buttressed by aerial bombardment and cross-border artillery fire, the ground personnel are advancing inside the Syrian territory along with tanks. President Recep Tayyip Erdogan had promised to wrap up the anti-Kurdish operations "very quickly".

"Our jets took off and started bombing. And now the ground operation is underway. Now we see how the YPG... are fleeing in Afrin. We will chase them. God willing, we will complete this operation very quickly," said the Turkish president. Concerns have been growing over the status of civilians in the region.

From the YPG's side, a representative named Nouri Mahmoudi said the Turkish forces were confronted. He said: "All the Turkish military's ground attacks against Afrin have been repelled so far and they have been forced to retreat."

The US has also urged Ankara to show some "restraint" before mounting a full-scale attack on the YPG fighters. "We urge Turkey to exercise restraint and ensure that its military operations remain limited in scope and duration and scrupulous to avoid civilian casualties," said the US State Department spokeswoman, Heather Nauert.